The present invention relates to a milling machine spindle extension having a self-contained pull-stud draw bar assembly.
There are two widely recognized problems within the profile milling industry. First, modem molds and dies currently include deeper cavities than were previously used. Standard milling tools are typically too short to construct such deep cavities Second, direct coupling of a milling tool to a host machine spindle may result in undesirable wear and abuse to that spindle during heavy milling operations.
Commonly attempted solutions to the two noted problems include spindle extensions. Two different types of spindle extensions are employed to alleviate the two problems. A long-reach spindle extension can help achieve deeper mold cavities. Surrogate spindle extensions help to minimize the wear and abuse to a host machine spindle bore during heavy milling. The surrogate extensions are typically much shorter and more durable than the long-reach extensions. Previously, both types of extension include the undesirable feature that, once attached to the machine spindle, the machine power draw bar tool-connecting apparatus is unusable. The latter feature renders spindle extensions cost inefficient relative to conventional milling directly from the host machine spindle.
A commonly used spindle extension, for both types of applications described above, is called a weldon shank spindle extension. There are several problems with weldon shank configurations stemming from the necessary use of set screws that maintain the tool in a secured position within the spindle extension. The set screws cause tool holder elongation that increases tool vibration and tends to hasten tool runout. Further, set screws tend to vibrate loose during heavy milling operations. Other problems with weldon shank configurations include the low percentage of surface contact between the tool shank and the spindle extension tool receiving bore, again causing excess vibration.
Other spindle extensions include collet systems in which a tool holder is secured by a draw bolt contained within the spindle extension. Draw bolt collet systems are inefficient because the entire extension must be removed from the host machine to access the draw bolt for tool changes. Other collet systems use exterior nuts to secure a tool in place but these have several disadvantages. A main disadvantage is that an exterior nut increases the diameter of the extension which results in a lower clearance profile, and possible inaccurate die making. A high clearance profile is essential when utilizing a long-reach spindle extension. A second disadvantage to the collet systems is that a relatively large amount of extra time and tools are needed for a machine operator to complete a tool change and resume a milling operation.
One other type of extension utilizes threaded tool connections. Threaded tool connections are solid, however, they also have disadvantages. The biggest difficulty involved with threaded tool connections is that the tool may seize within the extension due to the high torques associated with normal milling operation. Thus, threaded tool connections are not a viable alternative for the current milling industry.